This invention relates in general to control systems and in particular to an apparatus and method for self-calibration of a current feedback signal and a subsequent command signal within a control system.
Motor vehicles are becoming increasingly sophisticated, especially with regard to electronic controllers for various onboard systems. The electrical to mechanical interfaces for many of these systems include a coil that is used to displace a mechanical device, such as a valve armature with the displacement of the armature being a function of the current flowing through the valve. Several examples include solenoid valves that control vehicle brakes in Anti-Lock Brake, Traction Control and Vehicle Stability Control Systems, solenoid valves that control torsion rods in Active Suspensions Systems and coils that control steering assist in Electro-Hydraulic Steering Systems. Additionally, variable currents are used to drive solenoids, DC motors, and other inductive loads in many automotive subsystems. Furthermore, the position of linear solenoids and the torque in DC motors are directly correlated to the current drive, which require accurate current measurements for precise positioning.
There is shown in FIG. 1, a typical prior art vehicle control system 10 that utilizes a switch 12 to control the flow of electric current through a load 14. As shown in FIG. 1, the load 14 is connected between a first side of the switch 12 and a power supply V+. A second side of the switch 12 is connected to ground to provide short circuit protection to the power supply should the switch fail. As also shown in FIG. 1, a controller 16 is connected to the switch 12 that is operative to open and close the switch. The controller 16 typically includes a microprocessor with a memory that stores an operating algorithm. The controller 16 also is usually connected to one or more sensors that monitor operating parameters of the vehicle. The microprocessor, in accordance with the operating algorithm, is responsive to the sensor signals to selectively open and close the switch 12 to activate and deactivate the load 14.
As described above, the load 14 is often a coil 20, as shown in FIG. 2. Additionally, the switch 12 is usually a semi-conductor device, such as a Field Effect Transistor (FET) 22 having a drain connected to one end of the coil 20 and a source connected to ground. Again, the other end of the coil 20 is connected to the power supply V+. The gate of the FET 22 is usually connected to a control port of an Electronic Control Unit (ECU) 24 which functions as the controller 16 described above. Typically, the ECU control port will be either “low” at ground potential or “high” at a fixed voltage, such as five volts. When the control port is low, the FET 22 is in a non-conducting state and blocks current flow through the coil 20 while, when the control port is high, the FET is in a conducting state, allowing a potentially high current to flow through the coil.
In order to provide closed loop control of a system feedback is required. For the load 20 shown in FIG. 2, feedback of the actual current flowing though the coil is required to confirm that the control is attaining the desired current. Devices for providing current feedback in a high current circuit are typically called current shunts, which can come in many forms including metal bars, resistors or semi-conductor devices. A resistive shunt would be connected between the inductive load 20 and the switch 22 (not shown). The current could then be measured by measuring the differential voltage across the shunt and applying Ohm's Law to convert the voltage into a current. This is the common method to measure the current through a solenoid coil or DC motor. The disadvantage of using such devices is that they can be expensive, require a large amount of space and need a means of dissipating any heat created by the flowing current.
Recently, inexpensive integrated FET chips, which include internal circuitry that provides a current feedback signal have been developed. This feature is typically referred to as “diagnostic feedback,”. “current sense output,” or “mirror FET.” Such integrated FET's are referred to as “feedback-FET's” in the following. A feedback-FET 32 is shown if FIG. 3, where components that are the similar to components shown in FIG. 2 have the same numerical designators. In FIG. 3, when the feedback-FET 32 is in its conductive state, a voltage that is proportional to the current flowing between the drain and source of the FET 32 is generated at a current feedback terminal 34 on the FET. The current feedback terminal 34 is connected by a current feedback line 36 to a corresponding current feedback port 38 on the ECU 24.
As described above, feedback-FETs are not the only means of attaining current feedback for a circuit; however, the discussion from here forward will use a feedback-FET device in the descriptions and claims.
In some applications, multiple loads are supplied with power from a single FET, but controlled with individual control FET's with one of the control FET's associated with each of the loads. This may occur, for example, when a plurality of solenoid coils are used to control the application of hydraulic pressure in a vehicle electronic brake system, such as an Anti-Lock Brake System, a Traction Control System and/or a Vehicle Stability Control System. Other applications may include control of multiple fuel injectors in an engine control system and control of solenoid valves in active suspension systems and electrically assisted power steering systems.
A typical multiple load control application is shown in FIG. 4, where components that are similar to components shown in the preceding figures have the same numerical identifiers. As shown in FIG. 4, a high end of each of a plurality of loads, shown as coils, L1 though Ln, is each connected to the source terminal of a feedback-FET 32. The feedback-FET drain terminal is connected a power supply V+while the feedback-FET gate is connected to the ECU 24. The feedback-FET 32 also has a current feedback terminal 34 that is connected by a current feedback line 36 to a corresponding current feedback port 38 on the ECU 24. As also shown in FIG. 4, a low end of each of the loads, L1 though Ln, is connected to a drain terminal of an associated control, or driver, FET, T1 through Tn. The source terminal of each of the control FET's is connected to ground, while the gate of each of the driver FET's is connected to the ECU 24.
During operation of the control circuit shown in FIG. 4, the feedback-FET 32 is placed into its conducting state to provide power to each of the loads L1 through Ln. Each load is then individually energized by selectively placing the corresponding driver FET T1 through Tn into its conducting state. Often, only one load will be activated at a time, in which case the current sensed by the feedback-FET 32 will be the same as the load current. Thus, the use of a single feedback-FET 32 allows monitoring a plurality of loads while minimizing component costs.
While the use of a feedback-FET 32 can provide useful feedback information to an ECU 24, the particular application may require a high level of accuracy for the information. Unfortunately, the built-in amplifier in these devices may not provide the needed level of accuracy for a particular application. Accordingly, it would be useful to provide a self-calibrating capability for a feedback-FET.